That is just a fat fur.
Mainstream fur... Hmm, I guess rabid fan of furry who hopes one day to turn into their fursona, act in public a bit like their animal base for their fursona, and kinda blatantly force their behaviors in people faces to "express" who they are and "subtly" hint they're clearly a furry and people should embrace it.
That's just my interpretation. From my experience and first hand confrontation, it's what I see as a mainstream fur.
Then again, the term's very broad. It could just be a gateway branch to a number of subculture furs within the fandom. You got the I wanna be emo type furs that live with abusive, xenophobic parents, then you got the kinda slumpy, overweight ones with a fur suit who make it out to FurCon every chance they get. They're a little weird when talking with them and they smell funny. Then you got the net savvy porn artists who make a killing off the others with their commissions. There's all kinds really.
(Disclaimer as I know it's needed for the jumpy, quickly offended types. If you think you fall into those categories, don't get mad at me for pointing anything out. It's obvious as the sky is blue. I mean no rudeness behind my observations.)